I do not look at genocide as an evolutionary step in getting rid of less powerful people, and I don’t know anyone who isn’t a psychopath who would think that. Genocide is the result of many different psychological, cultural, social, economic, and other factors. It is humanity gone wrong.

Imagine that you are an atheist. Imagine that you are exactly as you are now, with your same emotions and thoughts, with the only difference being that you don’t believe there’s a God. Why would you object to genocide? Do you only object to genocide right now because you think God doesn’t like it? If God spoke to you tomorrow and said genocide is a good thing, would you say “okay God, if you say genocide is awesome, then I think it’s awesome too!” or would you be horrified and disbelieving? Would you have a strong negative emotional reaction to it? Because I’m pretty sure you would, and I know for a fact that I would.

I object to genocide because I think life is inherently valuable. Why is it inherently valuable? Because I personally think my life is valuable, because I can empathize with other human beings enough to realize that they also consider their lives to be valuable, and because I don’t believe in an afterlife. I think it is a terrible thing to take away someone’s life because there’s no coming back from it. I wouldn’t want that for myself, I wouldn’t want it for my loved ones, and because I am not an anti-social psychopathic monster, I don’t want that for other people, either. Atheists have the same emotions that you do, and emotions, combined with our personal interpretations of the values of whatever culture we’ve been socialized into, are the driving force behind morality. That is a simplification, but basically that is how I understand morality and where I think my own sense of ethics comes from.

I don’t really think in terms of evil, just because I think of evil as a supernatural force or being that causes bad things in the world, and I don’t think such a force or being exists. Other atheists will have their own interpretations, but if they talk about evil, they probably aren’t using that word in the same way that you do, and it doesn’t mean the same thing to them that it does to you.

The way you ask these questions makes me curious, do you think that all positive emotions are caused by a connection to God, and that if you don’t believe in God then you don’t have positive emotions? Atheists are not Vulcans.

I highly recommend that you read some posts by Greta Christina, a prominent atheist blogger.
Suffering, bad luck, and death:
http://freethoughtblogs.com/greta/2008/04/09/atheism-bad-luc/
http://freethoughtblogs.com/greta/2007/06/25/the-problem-of/
http://freethoughtblogs.com/greta/2010/01/04/atheism-death-pessimism-realism/

Explanations of atheists:
http://freethoughtblogs.com/greta/2009/03/31/myths-and-truths/
http://freethoughtblogs.com/greta/2007/03/27/how_i_became_an/